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Nuclear Physics (5Th Lecture)

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Nuclear Physics (5Th Lecture) Nuclear Physics Parity non-conservation in beta decay Parity was introduced first by E. Wigner in 1927, for describing (5th lecture) symmetry of atomic quantum states against r → -r transformation. Electromagnetic interaction conserves the parity Content Strong interaction conserves the parity It was assumed that the weak interaction also conserves the parity. • Parity violation in weak interaction, Wu experiment However, in 1956 T.D.Lee and C.N.Yang showed, that there was no experimental evidence for parity conservation in weak interaction. • History of the neutrino, leptons’ families. Leptonic charge They explained the so called „tau-theta puzzle” assuming that • Anti-neutrino detection (Reines-Cowan experiment) parity is not conserved → Nobel-prize in 1957! • Neutrino detection (Davis experiment) • Solar neutrino puzzle • Neutrino oscillation, and neutrino masses 1 2 If parity is not conserved in weak interaction, this should be seen Why does this mean parity violation? also in nuclear b-decay! • Direction of electrons: p (momentum): vector → -1 if mirrored Experimental proof: C.S.Wu et al. (1957) • Direction of angular momentum J r p : axial vector → +1 if mirrored Result: 60 Polarized Co source: 2μB n - small temperature (~3 mK), kT e if parity was conserved strong magnetic field n Mirroring the coordinate axis is μB kT not a good symmetry! 3 4 http://hyperphysics.phy-astr.gsu.edu/hbase/quantum/imgqua/wu.gif 1 Angular momenta Explanation of the parity violation The story of the neutrino in Wu experiment (complete violation) Early history: 0 1914: Chadwick discovered the continuous energy spectrum of 214 Pb (RaB) using a magnetic spectrometer ~ ~ Interpretation (Rutherford): monoenergetic electrons come ~ 0 out of the nucleus, but they loose energy in the matter Only right-handed antineutrinos (p↑↑s), and 1927: Ellis and Wooster experiment: total energy released by left handed neutrinos (p↑↓s) exist! 210Bi (RaE) b-decays, measured by a differential calorimeter, that could stop all electrons inside. W. Pauli: „Cannot believe that God is left-handed!” E0 = 1050 keV as calculated from Antineutrinos have to fly in the direction of the J 5 ! the mass difference So electrons have to fly in opposite direction (Sp ~0) Consequence for the H operator: Emeasured = 344±34 keV b Fermi, G.T. Hb = (Hb,S + Hb,V) + (Hb,A + Hb,T) Finally, the interaction operator: Hb = Hb,V + Hb,A 5 6 Not only energy was „missing” Double beta decay 14C14N e- 6 7 92Mo cannot beta-decay, since 92Nb is J: 0 → 1 + ½ (??) Angular momentum non-conservation? „higher” in energy. However, the energy 6He6Li e- of 92Zr is lower, so if the nucleus could Cloud chamber photo „jump” DZ=2, then it was energetically S. Szalay & Gy. Csikai (Hungary) favorable! Momentum non-conservation? Ideas for the solution of the puzzle: A A - ~ The ordinary double beta decay: Z X Z2Y 2e 2 N. Bohr, J.C. Slater, L.D.Landau: conservation laws are not It may occur in certain even-even nuclei. valid in microscopic scale, only statistically! (Hmm…) 19 22 It has been observed at 35 isotopes, with T1/2~10 -10 years 1930: W. Pauli: hypothesis of the „neutron”. A A - The neutrinoless double beta decay Z X Z 2Y 2e M<<Mproton , neutral, spin=1/2 It may occur only if the neutrino is its own antiparticle: „We will never detect it directly” (Pauli) A A - Z X Z1K e The neutrino emitted in the first, will 1933: E. Fermi renamed it „neutrino”, after the discovery of A A - Z1 K Z2Y e induce the second reaction Chadwick of the „massive” neutron (M~M ) proton 25 7 It has not been observed yet. T1/2 >10 years 8 2 By that time it was known (Dirac) that fermions should have Leptonic charge antiparticles. neutrino = antineutrino? The lack of neutrinoless double beta decay (and many other If no then we call them Dirac neutrino processes) led to the discovery of leptonic charge conservation law If yes, we call them Majorana neutrino Three „family” of the leptons It was discovered by 3 physicists in the same year (1953) - independently: 1936: C. D. Anderson: discovery of the muon (m ) (mm~200 me) 1962: Lederman, Schwartz, Steinberger: G. Marx (Hungarian) - 1953 January (published in German) discovery of the m-neutrino (Nobel-prize 1988) J. B. Zeldovich (Soviet) - 1953 July (published in Russian) 1975: M. L. Perl (SLAC, USA): H.M. Mahmoud and discovery of tau meson (mt~3500 me) (Nobel-prize: 1995) E.J. Konopinsky (USA) - 1953 November (published in English) 2000: DONUT experiment (Fermilab, USA) discovery of tau neutrino L Le Lm Lt The lepton families (flavours) The „family” leptonic charge = 1 for the particles, Charged lepton Mass Neutral lepton Mass -1 for the antiparticles, and - electron (e ) 1 me electron neutrino (e) ? 0 for the non-leptons. - muon (m ) ~200 me muon neutrino (m) ? A conservation law exists for the total leptonic charge! - (And for most reactions also for the family leptonic charge) tau meson (t ) ~3500 me tau neutrino (t) ? … + their antiparticles! 9 10 A few examples: Anti-neutrino detection (Reines-Cowan experiment) A A - ~ Negative b-decay Z X Z 1Y e big amount of interacting particles Very small interaction L easy detection of the reaction products 0 0 1 -1 e probability A X AY e low background Z Z -1 Positive b-decay ~ p n e 0 0 -1 1 Le water (~400 l) A - A Electron capture Detecting e+ → annihilation gammas Z X e Z -1Y 1 N 6,61022 3 (2 x 511 keV) 0 1 0 1 Le cm nuclear reactor Detecting n → high energy gammas - - ~ m m Pion decay (Savannah River) following n-capture in CdCl2 Many b- decays Cd(n,g) reactions 0 1 -1 Lm - - ~ from the fission m e e m Muon decay products 0 1 -1 Le 1 1013 2 1 0 0 1 Lm cm s 11 12 3 Results: 1953: 24 ±12 counts/h -43 2 exp 1,2 0,510 cm 1956: 2,88 ± 0,22 counts/h -43 2 1958: 36,4 ± 4 counts/h theory 1,0 0,1710 cm Nice agreement!! ☺ Neutrino detection (Davis experiment) The Reines-Cowan method does not work! - http://www.scienceinschool.org/repository/images/issue19neutrinos10_l.jpg n p e delay - No target Detecting e → not possible, electrons can be are everywhere prepared! Detecting p → not possible, protons are everywhere No source! 13 14 discriminators coincidence circuits A A - B. Pontecorvo: use nuclei as target: X Y e R. Davis, J. Bahcall Z Z 1 „Homestake” experiment Problem: big amount of X needed few atoms of Y created 1957 – 1994 How to separate and detect? (Nobel-prize 2002) X and Y must be Y should be radioactive 1478 m underground! very different chemically 37 37 - 17Cl18Ar e 37 - 37 - 18Ar e 17Cl (T½ = 35 days, e capture, EC) 1 4 C2Cl4 Source: Sun 41H2 He 2e 2 26,22 MeV 1 111 Production rate: 4,810 13,11 MeV J Solar constant: 1361 kW/m2 = 136,1 J/(cm2s) 1 The neutrino flux then: 4,81011 136,1 6,531013 cm 2s 15 http://www.bnl.gov/bnlweb/raydavis/images/hires/recovery_schematic.gif 16 4 R. Davis, J. Bahcall „Homestake” experiment 1 Neutrino oscillation Results: 1 SNU = 10 -36 37Cls Expected value ~ 9 SNU Main idea: neutrinos have masses, Solar neutrino puzzle Measured value ~ 3 SNU e, m, t are NOT mass-eigenstates! (B. Pontecorvo 1957) The weak interaction selects according to the „flavours” Creation according to flavours → mixed mass state Attempts to solve the puzzle: Propagation → according to masses → mixing changes • Experimental error? NO! Several other neutrino experiments Detection (Davis) → according to flavours again confirmed the results e U1,1 U1,2 U1,3 1 U U U • Error in the Sun model? (Pulsating Sun?) m 2,1 2,2 2,3 2 t U3,1 U3,2 U3,3 3 • Neutrino oscillations? (Yeah!) ☺ flavour mass eigenstates eigenstates MNS matrix (1962) 17 (Maki, Nakagawa, Sakata) 18 Neutrino oscillation (contd.) 2 2 2 For 3 neutrinos obviously Dm12 Dm23 Dm31 0 cos sin Simple example: 2 neutrinos e 1 m -sin cos 2 The e neutrino was created with p momentum at t = 0. mixing „angle” 2 2 2 4 Since Ek p c mk c - iE1t - iE 2t at t time we have: exp cos 1 exp sin 2 The probability to detect a e again after L ~ c∙t distance: L 2 2 E2 - E1 L Pt 1-sin 2sin c 2c E - E L Dm2 L Can be shown that: 2 1 1 ,27 , where D m 2 m 2 - m 2 [eV]2 2c pc 2 1 L [m], pc [MeV] 19 20 5 Neutrino oscillation (contd.) Nobel-prize in Physics 2015 Several experiments confirmed „for the discovery of neutrino oscillations, which shows that Super Kamiokande (Japan) Sun picture neutrinos have mass” taken with Kamland (Japan) CERN to Gran Sasso (Italy) 21 22 Nobel-prize in Physics 2015 Nobel-prize in Physics 2015 Arthur B. McDonald Takaaki Kajita 23 24 6 .
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